Many commercially important applications require laser radiation in parts of the optical spectrum for which there are no readily available efficient and compact laser diodes or solid-state lasers pumped by laser diodes. For example, tri-color (red, green and blue or RGB) lasers are desired for various visual display applications. The optimal wavelength range for the blue is between about 440 nm and 460 nm, for the green it is between 510 nm and 540 nm, and for the red it is between 610 nm and 630 nm. It is desirable that these lasers be compact and either be solid-state lasers or as a minimum comprise solid-state lasers pumping another solid-state laser material. A minimum optical output power level of greater than a watt for each of the colors is required for many applications.
One completely solid-state design uses a very efficient and powerful GaAlAs laser diode emitting close to 800 nm to pump a neodymium (Nd) ion incorporated in a host formed in a bulk rod or other structure. The neodymium ion will cause lasing at various wavelengths in the near infra-red, which wavelengths can then be frequency doubled into the visible spectrum using a non-linear crystal for second harmonic generation (SHG). The most popular host for neodymium is yttrium aluminum garnet (YAG) The most efficient line for Nd:YAG occurs at 1064 nm, the second harmonic of which produces an excellent green source at 532 nm. Nd:YAG will also lase at 946 nm. This wavelength, when frequency doubled to 473 nm, is slightly outside the desired blue wavelength band. However, if the YAG is replaced by a vanadate host, the transition is shifted to 914 nm, thus producing a second harmonic at the more desirable 457 nm. Unfortunately, the Nd transition that produces these low 900 nm wavelengths has a terminal lasing level which is partially populated at room temperature making the laser a more difficult system than one based on the stronger 1064 nm line or similar lines in other Nd host materials.
The red color presents a much greater problem than the blue. Nd:YAG has a transition at 1319 nm, which, when frequency doubled to 659 nm, is too red for high sensitivity by the human eye. Other possible hosts, such as vanadate or yttrium lithium fluoride (YLF), do not solve this problem. It is possible that III-V semiconductor lasers can be compositionally tailored for the desired wavelengths, but at the present time they emit outside of the desired range, especially when they are designed to output high power in the diffraction limited mode.
Another approach for attaining the red color uses sum-frequency techniques entailing different laser lines, or a sum frequency of one laser line and another frequency generated by a different process, such as an optical parametric oscillator (OPO). All known versions of this approach require lasers at multiple wavelengths or a single laser and multiple non-linear devices to produce the desired frequency.
Since solid-state Nd:YAG lasers operating at 1064 nm and pumped by diode lasers are now commercially available with diffraction-limited powers in excess of ten watts, it is highly desirable to use this type of laser in conjunction with a simple and efficient non-linear device to generate a coherent red beam in the wavelength range of 610 nm to 630 nm, and also for other particular wavelengths in the visible, IR and UV, especially in the desired 440 nm to 460 nm blue region.